Diffusion bonded cathode for electron discharge device

ABSTRACT

A method of fabricating cathodes for electron discharge devices wherein the cathodes comprise a hollow tubular stack and a substantially cup-shaped top cap fitted over one end of the stack. The method comprises thoroughly cleaning the stack and cap such as by degreasing and water washing and then fitting the cap to the stack and firing first in air at a temperature of about 500-600*C and then in hydrogen at a temperature of at least 1,100*C whereby a diffusion bond is formed between the cap and the stack.

United States Patent Decker et a1.

1451 Aug. 12, 1975 1 DIFFUSION BONDED CATHODE FOR OTHER PUBLICATIONSELECTRON DISCHARGE DEVICE B. R. Garrett et 211., Broad Applleatlons ofDiffusion [751 Inventors: John J. Decker; Donald Bonding," NASA CR409,March 1966, pp. 19-26.

Kerstetter, both of Emporium, Pa.

[73] Assignee: GTE Sylvania Incorporated, Primary ExaminerRoy LakeStamford, Conn. Assistant ExaminerJames W. Davie 1 Attorne A em, orFirmNorman J. OMalle [22] May 18,1973 Wi11iam 1-L 1V1cNei1hDona1d R.Castle y [21] Appl. No.: 361,830

[57] ABSTRACT [52] US. Cl. 29/25.l4; 29/494; 313/270 A method offabricating cathodes f electron [51] Int. Cl? I-IOIJ 9/00; 1323K 31/02charge devices wherein the cathodes comprise a 1 [58] Field of Search29/25.13, 25.14, 191, 484, 1 tubular Stack and a Substantially cupshaped top 29/498 488, 494; 313/270 cap fitted over one end of thestack. The method com prises thoroughly cleaning the stack and cap suchas 1 References Cited by degreasing and water washing and then fittingthe UNITED STATES PATENTS cap to the stack and firing first in air at atemperature 2,708,249 5/1955 Pryslak 29/25.16 X of about 500600C andthen in hydrogen at a tem 3 5. 75 3 9 3 Knauf, Jr 313 270 perature of atleast 1,100C whereby a diffusion bond 3,214,626 10/1965 Larson 313/270is formed between the cap and the stack. 3,333,141 7/1967 Lemmens etal..... 313/346 R 3,667,110 6/1972 Gwyn, Jr 29/488 x 2 Clams, 2 DrawlngFlgul'es FORM 5 TACK FORM TOP CAP DEGREA SE WATER WASH FIRE IN AIR FIREIN H PATENTEB AUG 1 2 I975 FORM STACK FORM TOP CAP DEGREASE WATER WASHFIT CAP TO STACK FIRE IN AIR FIRE IN H ji 9. a

DIFFUSION BONDED CATHODE FOR ELECTRON DISCHARGE DEVICE BACKGROUND OF THEINVENTION This invention relates to cathodes for electron dischargedevices and more particularly to cathodes for cathode ray tubes. Suchcathodes generally comprise a hollow tubular stack or body portionhaving a cupshaped top cap bonded or otherwise affixed to one endthereof. In the past it has been the practice to weld the top cap to thestack in one or more places. While this procedure has worked well formany years it is costly and time consuming and introduces other problemsas well. One of the major ones of these problems is the fact that theweld between a wall of the top cap and the stack is seldom smooth, andoften introduces burrs or protrusions which can subsequently break offduring tube operation. Furthermore, with the current trend toward highvoltage operation in cathode ray tubes any protrusions or unevennessformed on the cathode makes it very subject to disruptive arcs.

Therefore, it would be very advantageous if a method of bonding a topcap to the stack could be found which obviated the above-citeddisadvantages.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, an object of thisinvention to obviate the above-cited disadvantages of the prior artmethod.

It is another object of the invention to enhance the fabrication ofelectron discharge device cathodes.

Yet another object of the invention is the provision of a method ofattaching top caps to stacks which method does not utilize welding andwhich introduces no uneven spots or protrusions or weld contaminationinto the cathode assembly.

These objects are achieved in one aspect of the in vention by theprovision ofa method which utilizes diffusion bonding of the top cap tothe stack. To achieve this method it is necessary that the parts; i.e.,the top cap and the stack be extremely clean. This can be achieved byfirst degreasing the cathode top cap and stack and can also includewashing in water to remove any contaminating salts. Also, a high degreeof precision in the fitting of the parts is required. This degree ofprecision is accomplished by having the internal diameter of the top capthe same as or just slightly larger than the outside diameter of thestack.

After cleaning, the top cap is fitted over one end of the stack,preferably under pressure of about pounds. The now assembled cap andstack are then first fired in air to oxidize any volatile contaminantsthereon and then second fired in a reducing atmosphere, generallyhydrogen, at a temperature sufficiently high to form a diffusion bondbetween the top cap and the stack. While the oxidizing firing can takeplace at a temperature of about 500-600C the final or second firing inthe reducing atmosphere should be accomplished at a temperature of about1,100C for at least one minute. Under these conditions the top cap isfirmly bonded to the stack as by a diffusion bond.

The phenomenon being exhibited by this method has long been known toexist. That is, high temperature firing of super cleanparts hasgenerally resulted in a number of the parts sticking together. However,this in the past has always been a troublesome aspect of the firingprocedures and it is believed that this is the first known practical useof the phenomenon.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of acathode, with parts broken away which can be fabricated by this method.

FIG. 2 is a flow diagram of the basic process of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding ofthe present invention, together with other and further objects,advantages and capabilities thereof, reference is made to the followingdisclosure and appended claims in conjunction with the above-describeddrawings.

Referring now to the drawings with greater particularity, in FIG. 1there is shown a cathode assembly 10 which is comprised of a hollowtubular stack 12 and a top cap 14 bonded thereto. The top cap 14 isgenerally cup-shaped and fits tightly over one end of stack 12. It willbe observed from a study of the figure that no additional bondingmaterial is utilized between the two pieces. Only a tight fit and a highfiring temperature are necessary to achieve a thermal diffusion bond.

Both the stack and top cap are formulated or fabricated from more orless conventional cathode materials: that is, nickel or nickel alloyscontaining at least about nickel. The process works well with all knowncathode materials.

The method is accomplished by first forming the stack by any knowntubular forming technique; and the top cap also is fabricated by anyknown techniques such as die drawing. After the stack and cap are formedthey are cleaned, as by degreasing to remove any organic contaminatingmaterials and can then be water washed to remove any salts that may bethereon. Ultrasonic cleaning is also a desirable cleaning technique.After cleaning, the cap is fitted to the stack, and a tight fit ispreferable. Such a tight fit, of course, is accomplished by fabricatingthe internal diameter of the top cap so that it is the same as or onlyslightly larger than the outside diameter of the stack. After the capand stack are fitted together they are first fired in air to oxidize anyvolatile contaminants that may have been picked up and this firing isaccomplished at a temperature of about 500600C. After the oxidizingfiring the cathode assembly is fired in a reducing atmosphere such ashydrogen at a temperature sufficiently high to form a diffusion bondbetween the top cap and the stack. The temperature of this firing shouldbe about 1,100C for at least one minute and preferably about 3 minutes.After the final firing it is found that the top cap is diffusion bondedto the stack in an extremely clean manner wherein no protrusions orburrs have been introduced.

At this stage the cathode is ready to have emissive material appliedthereto.

It will be seen from the above that use of this process provides a verynovel method of fabricating cathodes which is simple and inexpensive andeliminates the necessity of welding the top cap to the stack. It is veryreadily automated and produces extremely clean cathodes that can beoperated in tubes employing extremely high anode voltages; that is, inthe neighborhood of 30,000 volts, without causing noticeable arcing.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be apparent to thoseskilled in the art that various changes and modifications can be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:

1. A method of fabricating cathodes for electron discharge devices, saidcathodes comprising a hollow tubular stack and a substantiallycup-shaped top cap fitted over one end of said stack and affixedthereto, said method comprising the steps of: forming said tubular stackfrom a nickel alloy containing at least about 95% nickel; forming saidtop cap from a nickel alloy containing at least about 95% nickel; saidtop cap having an internal diameter just large enough to fit over theexternal diameter of said stack; cleaning said stack and said top cap;drying said stack and top cap; fitting said top cap onto said stack toform a cathode assembly; firing said assembly in air to oxidize anyvolatile contaminants thereon at a temperature of above about 500600C;and firing said assembly in a reducing atmosphere at a temperaturesufficiently high to form a diffusion bond between said top cap and saidstack, said cathode assembly reaching a temperature of about 1,100C forat least one minute during said firing in said reducing atmosphere.

2. The method of claim 1 wherein said top cap is fitted to said stackwith a pressure of about 15 pounds.

1. A METHOD OF FABRICATING CATHODES FOR ELECTRON DISCHARGE DEVICES, SAIDCATHODES COMPRISING A HOLLOW TUBULAR STACK AND A SUBSTANTIALLYCUP-SHAPED TOP CAP FITTED OVER ONE END OF SAID STACK AND AFFIXEDTHERETO, SAID METHOD COMPRISING THE STEPS OF: FORMING SAID TUBULAR STACKFROM A NICKEL ALLOY CONTAINING AT LEAST ABOUT 95% NICKEL, FORMING SAIDTOP CAP FROM A NICKEL ALLOY CONTAINING AT LEAST ABOUT 95% NICKEL, SAIDTOP CAP HAVING AN INTERNAL DIAMETER JUST LARGE ENOUGH TO FIT OVER THEEXTERNAL DIAMETER OF SAID STACK, CLEANING SAID STACK AND SAID TOP CAP,DRYING SAID STACK AND TOP CAP, FITTING SAID TOP CAP ONTO SAID STACK TOFORM A CATHODE ASSEMBLY, FIRING SAID ASSEMBLY IN AIR TO OXIDIZE ANYVOLATILE CONTAMINANTS THEREON AT A TEMPERATURE OF ABOVE ABOUT500*-600*C, AND FIRING SAID ASSEMBLY IN A REDUCING ATMOSPHERE AT ATEMPERATURE SUFFICIENTLY HIGH TO FORM A DIFFUSION BOND BETWEEN SAID TOPCAP AND SAID STACK, SAID CATHODE ASSEMBLY REACHING A TEMPERATURE OFABOUT 1,100*C FOR AT LEAST ONE MINUTE DURING SAID FIRING IN SAIDREDUCING ATMOSPHERE.
 2. The method of claim 1 wherein said top cap isfitted to said stack with a pressure of about 15 pounds.